This invention relates to a decoder for receiving, decoding and conversion of frequency domain encoded signals, e.g. MPEG-2 encoded video signals, into standard output video signals, and more specifically to a decoder which formats an encoded high resolution video signal to a decoded output video signal having a user-selected resolution.
In the United States a standard, the Advanced Television System Committee (ATSC) standard defines digital encoding of high definition television (HDTV) signals. A portion of this standard is essentially the same as the MPEG-2 standard, proposed by the Moving Picture Experts Group (MPEG) of the International Organization for Standardization (ISO). The standard is described in an International Standard (IS) publication entitled, xe2x80x9cInformation Technologyxe2x80x94Generic Coding of Moving Pictures and Associated Audio, Recommendation H.626xe2x80x9d, ISO/IEC 13818-2, IS, 11/94 which is available from the ISO and which is hereby incorporated by reference for its teaching on the MPEG-2 digital video coding standard.
The MPEG-2 standard is actually several different standards. In MPEG-2 several different profiles are defined, each corresponding to a different level of complexity of the encoded image. For each profile, different levels are defined, each level corresponding to a different image resolution. One of the MPEG-2 standards, known as Main Profile, Main Level is intended for coding video signals conforming to existing television standards (i.e., NTSC and PAL). Another standard, known as Main Profile, High Level is intended for coding high-definition television images. Images encoded according to the Main Profile, High Level standard may have as many as 1,152 active lines per image frame and 1,920 pixels per line.
The Main Profile, Main Level standard, on the other hand, defines a maximum picture size of 720 pixels per line and 567 lines per frame. At a frame rate of 30 frames per second, signals encoded according to this standard have a data rate of 720*567*30 or 12,247,200 pixels per second. By contrast, images encoded according to the Main Profile, High Level standard have a maximum data rate of 1,152*1,920*30 or 66,355,200 pixels per second. This data rate is more than five times the data rate of image data encoded according to the Main Profile Main Level standard. The standard for HDTV encoding in the United States is a subset of this standard, having as many as 1,080 lines per frame, 1,920 pixels per line and a maximum frame rate, for this frame size, of 30 frames per second. The maximum data rate for this standard is still far greater than the maximum data rate for the Main Profile, Main Level standard.
The MPEG-2 standard defines a complex syntax which contains a mixture of data and control information. Some of this control information is used to enable signals having several different formats to be covered by the standard. These formats define images having differing numbers of picture elements (pixels) per line, differing numbers of lines per frame or field and differing numbers of frames or fields per second. In addition, the basic syntax of the MPEG-2 Main Profile defines the compressed MPEG-2 bit stream representing a sequence of images in five layers, the sequence layer, the group of pictures layer, the picture layer, the slice layer, and the macroblock layer. Each of these layers is introduced with control information. Finally, other control information, also known as side information, (e.g. frame type, macroblock pattern, image motion vectors, coefficient zig-zag patterns and dequantization information) are interspersed throughout the coded bit stream.
Format conversion of encoded high resolution Main Profile, High Level pictures to lower resolution Main Profile, High Level pictures; Main Profile, Main Level pictures, or other lower resolution picture formats, has gained increased importance for a) providing a single decoder for use with multiple existing video formats, b) providing an interface between Main Profile, high level signals and personal computer monitors or existing consumer television receivers, and c) reducing implementation costs of HDTV. For example, conversion allows replacement of expensive high definition monitors used with Main Profile, High Level encoded pictures with inexpensive existing monitors which have a lower picture resolution to support, for example, Main Profile, Main Level encoded pictures, such as NTSC or 525 progressive monitors. One aspect, down conversion, converts a high definition input picture into lower resolution picture for display on the lower resolution monitor.
To effectively receive the digital images, a decoder should process the video signal information rapidly. To be optimally effective, the coding systems should be relatively inexpensive and yet have sufficient power to decode these digital signals in real time. Consequently, a decoder supporting multiple formats must minimize processor memory.
The present invention is embodied in a digital video decoder which operates in first and second modes. The digital video decoder receives an encoded video signal at a terminal, and includes decoding circuitry which operates in 1) a first mode to decode the received encoded video signal to provide a decoded video signal having a first spatial resolution; and 2) a second mode to provide the decoded video signal having a second spatial resolution which is less than the first spatial resolution. The digital video decoder further includes: formatting circuitry which includes a spatial filter that, when the decoding circuitry operates in the second mode, is responsive to a formatting control signal to process the decoded video signal to change the spatial resolution of the decoded video signal to produce at least one display video signal having a respective display spatial resolution, different from the first and second spatial resolutions; and a controller for switching the decoder between the first and second modes and for providing the formatting control signal to the formatting circuitry.